The method of a stepwise differential isothermal analysis (SDIA) has been designed for studies of reduction processes in metallurgical
systems. The basis of the method is in multi-parameter control of reaction rate and its use for monitoring temperature, heating
rate, and gas-phase composition. In this study hydrogen reduction of MoO3 and its mix with 30% Fe have been carried out using the SDIA technique. During the measurements, TG, DTG, DTA, EGA, and temperature
control are carried out. Kinetics parameters were determined and possible reduction mechanism was suggested. The SDIA technique
is well suited for these studies.
This paper describes a study on some physico-chemical properties of CuO−Bi2O3 mixed oxides of various composition and their reactivity during hydrogen reduction in the range 290–460°C. Depending on the
composition, the changes in the morphology of the samples, their specific surface areas, phase composition of Bi2O3 as well as the change in the amount of chemisorbed surface oxygen (specific surface oxidation ability) were found. The last
mentioned parameter is strongly affected positively or negatively (depending on the dose absorbed) by the pre-irradiation
with gamma-rays and accelerated electrons. Either reduction of CuO or consecutive reduction of both components with a maximum
rate on the surface or sub-surface layers of grain, proceed in different temperature ranges.
The retarding effect of liquid bismuth is partially compensated for by the sponging and trapping effects, so that the overall
reduction rate changes non-monotonously with the composition of the samples. Pre-irradiation leads in all cases to the lowering
of reduction rate, which can be correlated with the increase in the concentration of strongly bound oxygen forms, creating
centres of donor hydrogen chemisorption. The effect of gamma-radiation appears to be function of the threshold dose absorbed.
The physico-chemical properties and reactivity tested by hydrogen reduction have been studied for two series of NiO-ZnO mixed oxides of various composition. The solid nickel oxide or zinc oxide in interaction with the solution of nitrate of the second component were used as the precursors in each series. The differences in some physico-chemical parameters of the samples in both series were correlated with their reduction behaviour, followed both in iso- and non-isothermal regime. Moreower, the influence of various factors modifying the reactivity of mixed oxides was also investigated and the results were compared with those obtained from earlier studied analogous systems of quite different origin.
Some physico-chemical properties of NiO–ThO2 mixed oxides of various compositions have been investigated. The presence of strongly bound constitutional water in the hydrogel
of reactive forms of thorium dioxide, determined by their origin (thermal decomposition of mixed hydroxides) caused the different
reduction behaviour as compared with other mixed oxide systems containing the only, thermodynamically less stabile reducible
component. The significant effects of the thermal treatment in oxygen atmosphere, pre-irradiation by the gamma rays or accelerated
electrons under various conditions (in air or in water suspension) as well as of surface chemical activation with a platinum
complex on the reactivity of mixed oxides or reoxidized samples during their hydrogen reduction have been also proved.
The kinetics of reduction at relatively low temperatures with hydrogen of pure and doped metastable non-stoichiometric magnetite
with 1 at% Mn, Co, Ni and Cu and also with 5 at % Ni and Cu have been investigated by using isothermal thermogravimetry in
the temperature range 300–400°C. With increase in the concentration of the dopant (5 at% Ni and Cu), the reactivity increases.
The activation energies for pure magnetite varies from 7 to 9 kcal/mole with the preparation temperature of precursorf Fe2O3 (250–400°C), being the lowest for those prepared at the lowest temperatures. The corresponding activation energies for the
reduction of doped samples (Fe, M)3−zO4, it depends, apart from their porosity and surface areas, on the nature of the solute atom, amount of disorder, whether it
occupies the tetrahedral (A) or octahedral (B) sites in the non-stoichiometric spinel and possibly on hydrogen ‘Spill over’
Some physico-chemical properties and reactivity in their reduction with hydrogen of NiO—Y2O3 mixed oxides prepared in a dry way have been studied using isothermal thermogravimetry in the range of 320–410°C and temperature-programmed reduction. It was found that addition of small amounts of chloride and acetate anions retarded the reduction of nickel oxide and accelerated the reduction of mixed oxides. The presence of oxalate and formate ions manifests itself by a small positive effect. Introduction of platinum activator or heat treatment of the samples in various atmospheres led to a pronounced increase in the reduction rate. The efficiency of the spill-over effect increases with increasing proportion of non-reducible Y2O3. The pre-irradiation of the samples by accelerated electrons and gamma rays at a dose of 1 MGy results in a negative kinetic effect only with the samples containing an excess of nickel oxide.
Authors:T. Hashizume, K. Terayama, T. Shimazaki, H. Itoh, and Y. Okuno
We succeeded in studying the mechanism of hydrogen added carbothermic reduction process of iron-manganese oxide by means of
the new technique, simultaneous measurement of evolved gas analysis (EGA) and humidity sensor (HS). Water vapor evolved by
the reduction with hydrogen can be detected by HS. Other gas was detected by TCD. Without carbon, the hydrogen reduction process
was followed to the formation of the intermediate product between MnO and FeO and finally reduction to the mixture of MnO
and Fe. With carbon, the intermediate products between MnO and FeO was formed at about 780 K. The methane was formed in higher
temperature than 1073 K and the reduction with carbon proceeded mainly. At higher temperatures, methane decomposed to yield
nascent carbon that tended to result in the acceleration of the reduction rate with carbon. The study is concerned with the
mechanism of the hydrogen reduction of MnFe2O4 and the effect of without and with carbon on this reduction by means of combining EGA and HS.
Thermodynamic calculations predict the formation of hydrochloric acid gas and alkali tungstates during hydrogen reduction
of WO3 doped with alkali chlorides MCl (M=Li, Na, K). The formation of HCl was proved experimentally by simultaneously coupled TG-MS
measurements from RT to 1200C. The formation of HCl is the result of the reaction between MCl, WO3 and water. Ubiquitous traces of moisture in the gas are sufficient for reaction according to WO3+(2+2n)MCl +(1+n)H2O→M2+2nWO4+n+(2+2n)HCl (n=0, 1, 2). Laboratory reduction tests showed that the formed tungstates differ. NaCl and KCl form monotungstates (n=0), while LiCl produces more lithium-rich compounds (n=1, 2). Temperature and humidity, among other process factors, control subsequent reduction of the tungstates to metals.
Temperature programmed hydrogen reduction studies have been carried out for SnO2and Ce-Sn mixed oxides with and without Pd metal impregnation, to demonstrate the existence of spillover of hydrogen from
Pd metal centers to support oxides. TPR pattern of SnO2 showed a main peak at ~973 K indicating the bulk reduction of this sample. In Pd metal impregnated sample, the bulk reduction
peak shifts to lower temperature (~923 K) due to the spillover of activated hydrogen from Pd metal to SnO2 at relatively lower temperatures and its subsequent reaction with SnO2. For Pd impregnated Ce-Sn mixed oxide samples also, a similar effect or an enhanced reduction was observed indicating the
spillover effect of hydrogen. These results have been further confirmed from 119Sn Mössbauer spectroscopic measurements carried out for some representative samples of SnO2 and Pd/SnO2 heated in hydrogen flow up to a temperature of 473 K. The value of Sn2+/(Sn4++Sn2+) ratio was found to be significantly higher for Pd impregnated sample. Both these observations provide direct evidence for
the existence of spillover effect of hydrogen taking place in the metal impregnated samples.